1. Field of the Invention
The present invention relates to a strain wave gear apparatus including a rigid internal gear, a flexible external gear, and a wave generator.
2. Description of the Related Art
In general, a strain wave gear apparatus is known as a speed reducer which is lightweight and compact, and performs speed reduction at a high speed reduction ratio in a single step. A general strain wave gear apparatus includes an annular rigid internal gear, a flexible external gear arranged on an inside of the annular rigid internal gear, and a wave generator for partially meshing the flexible external gear with the rigid internal gear at two meshing positions by deflecting the flexible external gear in a radial direction, and moving the two meshing positions in a circumferential direction.
A typical conventional wave generator includes elliptical cams and elastically deformable special bearings respectively fitted into outer peripheral surfaces of the cams and each having a thin inner race and a thin outer race. Thus, there has been a strong demand for a wave generator which does not require the special bearings. In order to meet such a demand, there has been proposed a wave generator which uses general bearings (Japanese Patent Application Laid-Open No. 2009-222168).
However, in the wave generator disclosed in Japanese Patent Application Laid-Open No. 2009-222168, two eccentric rollers are arranged while overlapping with each other in a direction of an input rotation axis. Thus, the two meshing positions of the rigid internal gear and the flexible external gear are displaced relative to each other in a direction of the input rotation axis as viewed in a direction orthogonal to the direction of the input rotation axis. With this structure, when a load torque is applied to an output shaft, the load torque cannot be applied equally to the two meshing positions, which has hindered an increase in allowable load torque.
Further, when the load torque is applied to the output shaft, forces are generated respectively at the two meshing positions owing to a pressure angle of the teeth meshed with each other, and those forces are applied as radially inward forces to the wave generator. As described above, the two meshing positions are displaced from each other in the direction of the input rotation axis. Thus, vectors of the radially inward forces generated respectively at the two meshing positions are not opposed to each other, and a rotational moment is generated about a center of gravity of the wave generator. Therefore, when the load torque applied to the output shaft becomes higher, the rotational moment applied about the center of gravity of the wave generator also becomes higher, which has caused a problem of shortening of a life of the strain wave gear apparatus.